Optical waveguide delta-beta couplers have at least two optical fibers that couple light at from one optical fiber into the other. Each fiber includes a core and a cladding. In a coupler the claddings of each fiber touch each other along a selected, coupling region. The coupling of light from one fiber into another is more efficient when the clads of the two fibers are closer together. Coupling also depends upon the relative sizes of the two cores and clads. So, it is known to have cores and clads of equal sizes as well as cores and clads of unequal size depending upon the wavelengths selected for coupling.
During the formation of a coupler, the core of the optical fiber is substantially reduced in size. Part of the formation requires necking down the size of two or more of the coupled optical fibers to a diameter about the size of one of the fibers. Methods for forming such a necked down coupler is shown and described in U.S. Pat. Nos. 4,799,949 and 5,011,251 issued to Corning, Inc.
There are known techniques for altering the index of refraction of a portion of the coupler. These techniques include stretching or bending the coupler, especially in the coupling region. It is known that stretching or bending a fiber will alter its index of refraction. So, a coupler can be made into an optical switch by applying mechanical force. Application of an electromagnetic field to the coupling region also alters the index of refraction. However, when the applied mechanical force or electromagnetic field is removed, the coupler returns to its original index of refraction.
Germania is a common dopant for increasing the index of refraction of the cores. It is normally not used in the cladding where the index of refraction is less than the index of the core. Germania is also a photosensitive glass and changes its index of refraction upon exposure to ultraviolet light. The latter property of germania is useful in Bragg gratings that use optical fiber with core regions treated with ultraviolet light. A Bragg grating has a wavelength selective core comprising alternate regions of different indexes of refraction spaced apart by half the desired filter wavelength (in the glass). The Bragg gratings do not couple light from one fiber to another, but rather filter one or more wavelengths of light.
It is very difficult to precisely manufacture couplers. The coupling of light between fibers depends upon numerous manufacturing variables, including the amounts of dopants in the cores, the sizes of the cores and cladding, the reductions of the relative sizes of the fibers during a neck down process, the length of the coupling region, etc. Couplers are precision devices which are useless if they do not accurately couple the selected wavelength of light from one fiber to the other. However, after the fiber is fabricated, the coupling cannot be permanently changed to correct for manufacturing defects. Accordingly, there is a long felt need for a coupler that can permanently correct manufacturing defects without reliance upon continuous external mechanical forces or applied electromagnetic fields.